Previously, molybdenum and/or tungsten have been necessary alloying elements to provide a bainitic structure in steels under continuous air cooling conditions. Such alloying elements are scarce and expensive. Consequently such costly prior art bainitic steels, for example those containing molybdenum or molybdenum and boron, have found only limited commercial application.
Recently there have been developed ferrite/martensite and lower bainite/martensite dual phase steels, alloyed with Mo, Nb, V, Ti, B and rare earth metals (REM), produced by controlled-rolling or continuous annealing in the ferrite/austenite two-phase region after rolling. Such steels present practical problems such as the need for special equipment, complex manufacturing processes, and associated high production cost as well as high materials cost.
The present invention provides economical, tough, high strength low alloy (HSLA) C-Si-Mn-B steels having a bainitic microstructure on air cooling from a hot working temperature.